This invention relates to a semiconductor laser emitting device and, more particularly, to a phase-shift DFB (Distributed Feed-Back) semiconductor laser emitting device appropriate to a digital optical transmission system.
The semiconductor laser emitting device is an essential component in the conventional digital optical transmission system. A quarter-wavelength phase-shift DFB semiconductor laser emitting device has a multiple-layered semiconductor structure, which has diffraction gratings different in phase from each other by a half period at the central point of the laser oscillator. The quarter-wavelength phase-shift DFB is stable in single oscillation mode, and is appropriate to the digital optical transmission system. The multiple-layered semiconductor structure for the quarter-wavelength phase-shift DFB semiconductor laser emitting device is known to person skilled in the art. A typical example of the multiple-layered semiconductor structure is disclosed in xe2x80x9cSemiconductor Laserxe2x80x9d, edited by the Society of Applied Physics and published by Ohm Corporation, page 272, FIG. 12xe2x80x9412, 1994.
FIG. 1 shows a typical example of the phase-shift DFB semiconductor laser emitting device. The prior art phase-shift DFB semiconductor laser emitting device 41 has a multiple-layered semiconductor structure, which consists of a semiconductor substrate 2 of n-type InP, an optical guide layer 5 of n-type InGaAs grown on the semiconductor substrate 2, a multiple quantum well layer 6 grown on the optical guide layer 5, a clad layer 7 of p-type InP grown on the multiple quantum well layer 6 and a cap layer 8 of p-type InP grown on the clad layer 7. The cap layer 8 is overlain by a p-type electrode 9, and an n-type electrode 10 is formed on the reverse surface of the semiconductor substrate 2. Low-reflection coating layers 45/46 are formed on end surfaces of the multiple-layered semiconductor structure. The optical guide layer 5 serves as a laser resonator, and diffraction gratings 43/44 are formed at the boundary between the semiconductor substrate 2 and the optical guide layer 5. The diffraction grating 43 is different from the other diffraction grating 44 by a half of the period at a phase shift point 48. The prior art phase-shift DFB semiconductor laser emitting device 41 oscillates at the Bragg wavelength xcexB, and the Bragg wavelength xcexB is expressed as
xcexB=2xcex9neff
where xcex9 is the period of the diffraction grating and neff is the effective refractive index. As a result, the other oscillation mode is effectively restricted.
Although the phase shift point 48 is indispensable for the phase-shift DFB semiconductor laser emitting device, the internal electric field is made extremely strong around the phase shift point 48. When the bias voltage is increased, the lack of uniformity in the internal electric field gets more serious, and the output optical power is decreased. Moreover, the reflectivity of the low-refractive coating layers 45/46 is of the order of 1 percent, and the laser light is radiated in the backward direction as much as the laser light radiated in the forward direction. This means that the prior art phase-shift DFB semiconductor laser emitting device can not achieve a high optical output efficiency in terms of the input current.
Another prior art phase-shift DFB semiconductor laser emitting device is disclosed in Japanese Patent Publication of Unexamined Application No. 4-100287, and aims at a high optical output efficiency. The diffracting gratings are not changed at any phase-shift point in the prior art phase-shift DFB semiconductor laser emitting device disclosed in the Japanese Patent Publication of Unexamined Application. The period of the diffraction grating is gradually modulated in a part of the laser oscillator, and the gradual modulation is designed to be equivalent to the quarter-wavelength phase shift. The concentration of internal electric field is less serious rather than that of the prior art phase-shift DFB semiconductor laser emitting device shown in FIG. 1. This results in improvement of the intensity of the output laser light.
Thus, the prior art phase-shift DFB semiconductor laser emitting device disclosed in the Japanese Patent Publication of Unexamined Application achieves a high optical output. However, the end surfaces are coated with low-refractive coating layers, and the laser light is equally radiated from both end surfaces. This means that the prior art phase-shift DFB semiconductor laser emitting device still does not achieve a high optical output efficiency in terms of the input current.
Yet another prior art phase-shift DFB semiconductor laser emitting device is disclosed in Japanese Patent Publication of Unexamined Application No. 61-216383. A cleavage plane or a high refractive coating layer are used at the backward end surface of the prior art phase-shift DFB semiconductor laser emitting device, and the phase-shift point is closer to the backward end surface. The phase-shift point divides the laser oscillator at 3:7-4:6. The forward end surface is coated with a low-refractive coating layer. When the backward end surface is the cleavage plane, the reflectivity is of the order of 30 percent. However, the high-reflective coating surface achieves the reflectivity more than 90 percent. This results in that the prior art phase-shift DFB semiconductor laser emitting device radiates the laser light in the forward direction much more than the laser light radiated in the backward direction. The Japanese Patent Publication of Unexamined Application insists that the phase-shift point closer to the backward end surface enhances the stability of the single oscillation mode. The prior art phase-shift DFB semiconductor laser emitting device achieves a high optical output efficiency in terms of the input current. However, the period of the diffraction grating is changed at the phase-shift point as similar to the prior art phase-shift DFB semiconductor laser emitting device shown in FIG. 1, and the phase-shift point makes the internal electric field extremely strong thereat. Although the intensity of the output laser light is slightly enhanced by virtue of the phase-shift point farther from the forward end surface than the phase-shift point 48, the intensity of the output laser light does not satisfy the user.
The present inventor calculates the ratio of the forwardly output laser light to the backwardly output laser light on the assumption that the reflectivity at the backward end surface is 95 percent. The ratio is 4:1. When the cleavage plane is used for the backward end surface, the margin is only several microns. If the cleavage plane is deviated by several microns, the optical output is clearly decreased. This results in a serious distribution of the laser output characteristics between the products.
Still another prior art phase-shift DFB semiconductor laser emitting device is disclosed in Japanese Patent Publication of Examined Application No. 6-66509, which is corresponding to Japanese Patent Publication of Unexamined Application No. 60-125882. The prior art phase-shift DFB semiconductor laser emitting device aims at enhancement of the single oscillation mode. However, the prior art phase-shift DFB semiconductor laser emitting device can not increase the optical output efficiency in terms of the input current. The prior art phase-shift DFB semiconductor laser emitting device has two diffraction gratings different in period from one another in the laser oscillator, and the two diffraction gratings are asymmetrical with each other. The periods of the two diffraction gratings are designed in such a manner that the longitudinal mode with the minimum gain in one of the diffraction gratings is overlapped with the longitudinal mode with the minimum gain of the other diffraction grating. Although the two diffraction gratings enhances the single oscillation mode, they can not increase the intensity of the output optical light, and, accordingly, the Japanese Patent Publication of Examined Application is silent to any increase of the output optical light.
It is therefore an important object of the present invention to provide a DFB semiconductor laser emitting device, which is stable in the single oscillation mode, large in output optical power, high in current-to-laser light converting efficiency and small in dispersion of the device characteristics.
In accordance with one aspect of the present invention, there is provided a distributed feedback semiconductor laser emitting device for radiating a laser light beam comprising a pair of electrodes applied with a potential difference, a multiple-layered semiconductor structure connected to the electrodes of the pair and including a first compound semiconductor layer extending between a first end surface of the multiple-layered semiconductor structure serving as an optical output and a second end surface of the multiple-layered semiconductor structure opposite to the first end surface and a diffraction grating structure held in contact with the first compound semiconductor layer, extending between the first end surface and the second end surface and having a standard period region constant in period and connected at one end thereof to the first end surface and a modulated period region different in period from the standard period region and making the diffraction grating asymmetrical with respect to a center of the diffraction grating, a first reflective coating layer formed on the first end surface and a second reflective coating layer formed on the second end surface and making the first compound semiconductor layer serve as a resonator.